When is a version considered outdated?

[Tekime]

When is a version considered outdated?

It's common to have many small updates that don't significantly change the software itself. But sometimes there is a major rewrite or significant upgrade, and we will no longer fully support the older version. A version is considered officially outdated when it's major version number changes. However, we strongly recommend upgrading to all new versions to remain secure and get the latest features.

[SKNX3]

How come old versions have a harder time for upgrades. For example, I can't get my sites format into the latest version. Seo script 77